1. Field of the Invention
The present invention relates to biaxially oriented polyester films and their production methods. More specifically, it relates to biaxially oriented polyester films that are high in rigidity in all directions within the film plane, high in dimensional stability, and resistant to deformation under load, and serves as a base film for high density magnetic recording media that shows particularly high travelling durability when used as data-recording tape and improved preservability in tape's service environment, and it also relates to their production methods.
2. Description of the Prior Art
Recent magnetic recording tapes have become thinner and higher in recording density to permit the production of smaller products with longer recording time, and there are increased demands for tape products with smaller elongational deformation under tensile stress and longer preservability in tape's service environment. Under such conditions surrounding the development of magnetic recording tape products, there are increased demands for improved base film materials that are higher in strength as well as form stability and dimensional stability in the tape's service environment.
To provide base films that meet the above requirements, aramid materials have been used conventionally as they are high in strength and dimensional stability. Though they are high in price and disadvantageous in terms of cost, manufacturers have had to use them as there are no alternatives. On the other hand, in conventional methods for producing a high-strength biaxially oriented polyester film, film is once stretched in two directions, i.e., machine and transverse directions, and further stretched in the machine direction to ensure high strength in the machine direction (for example, JP-B-SHO 42-9270, JP-B-SHO 43-3040, JP-A-SHO 46-1119 and JP-A-SHO 46-1120). For additional increase in strength in the transverse direction, "longitudinal and transverse re-stretching processes" in which film is first re-stretched in the machine direction and then re-stretched in the transverse direction have been proposed (for example, such films are proposed in JP-A-SHO 50-133276 and JP-A-SHO 55-22915). High-strength polyester films produced by these conventional methods have such disadvantages as: 1) tape breaks during use, 2) insufficient rigidity in the transverse direction causes edge damage, 3) elongational deformation under stress or dimensional deformation due to environmental conditions result in a shift of recording tracks that cause errors when records are read out, and 4) insufficient strength brings about difficulty in thickness reduction and makes it impossible to achieve required magnetic conversion performance. Thus, many problems still remain to be solved to allow large-capacity, high-density magnetic recording tape to be produced from these films.
Further, another stretching method has been proposed in which preliminary stretching is performed prior to the above-mentioned stretching-orientation process. U.S. Pat. No. 5,409,657, for example, proposes a process in which film is subjected to preliminary stretching at a draw ratio of 1.2 to 3 times in the machine direction at temperatures of (polyester's glass transition temperature Tg+40).degree. C. to (crystallization temperature Tc-20).degree. C., followed by stretching in the transverse direction and the machine direction, and shows films that are strengthened only in the longitudinal direction. Further, JP-A-HEI 9-300455 also proposes (a) a process in which film is subjected to preliminary stretching at a draw ratio of 1.5 to 2.5 times in the transverse direction at temperatures of 100.degree. C.-120.degree. C., followed by stretching in the transverse direction and the machine direction, and (b) a process in which a preliminary stretching is performed at a draw ratio of 1.1 to 2.2 times in the machine direction at temperatures of 100.degree. C.-120.degree. C. in addition to the above-described process (a), and shows films that are strengthened only in the transverse direction. Furthermore, JP-A-SHO 58-145421 proposes a process in which film is stretched in the two directions simultaneously at a temperature of 115.degree. C. or higher, followed by simultaneous biaxial stretching, with the aim of producing thin films and increasing the production speed, and shows films with small Young's modulus. Films produced by these technologies, however, are not high in rigidity in all directions and cannot solve the problems associated with applying the material to the production of high-density recording tape.